Mechanisms of transduction of growth signals in normal and malignant cells are becoming clearer through analysis of cytoplasmic kinases that are activated by these signals. Alterations of these kinases are a frequent consequence, and possibly also a cause, of cancer formation. One partially characterized cascade of kinases results in activation of MAP kinase, resulting in mitogenic stimulation and some aspects of oncogenic transformation. In addition, a kinase homologous to MAPK has been identified with a different, and apparently opposing role in cell signaling. This kinase is activated in response to cell stressors, including environmental cytotoxic agents such as UV light. Because of this pattern of activation, this kinase has been termed the Stress Activated Protein Kinase, SAPK. Dr. Templeton's group has recently identified and reported a new three-kinase cascade that begins with the kinase termed MEKK, that activates SAPK by phosphorylation of the immediate activator of SAPK, termed SEK1. Operative mechanisms of this cascade (excepting those just described) are wholly unexplored. However, since signaling through SAPK involves hormonal and physical mediators that provocatively inhibit cell growth and promote apoptosis, this new cascade could represent signaling highly significant to carcinogenesis In this application l propose to study the mechanism by which MEKK becomes activated in response to cell stress signals. In particular, l will continue work to identify and characterize post-translational modification (i.e. phosphorylation) that regulates MEKK activity, and will also study the importance of MEKK interactions with itself as a homodimer and with other, yet unidentified cell proteins.